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arXiv:2608.04112

Poled-fibre phase modulator for efficient high-dimensional quantum measurements

Nayda Guerrero, Nelson Villalba, Gustavo H. dos Santos, Carlos J. Salazar, Cristobal Melo, Fernando Castillo, Jaime Cariñe, Guilherme B. Xavier, Esteban S. Gómez, Stephen P. Walborn, João Pereira, Gabriel Saavedra, Gustavo Lima

quant-phSubmitted Aug 4, 2026• Announced Aug 6, 2026• 1 citation

Abstract

Efficient detection of quantum states underpins advanced device-independent quantum-information protocols that provide the ultimate level of security for tasks including quantum random number generation and quantum key distribution (QKD). High-dimensional encoding is a natural route to boost the performance of such protocols, offering enhanced noise resilience and higher information capacity, yet their practical implementation remains challenging. A key experimental bottleneck in higher dimensions is the typical need of active modulators for basis selection, which incur substantial optical losses and polarization-sensitive operation. Poled optical fiber phase modulators (PFPMs) are a fiber-native electro-optic technology that naturally addresses these challenges, combining sub-dB insertion loss, intrinsic polarization independence, and direct compatibility with standard telecommunications fiber. Here we report the first use of a PFPM for active quantum-state measurements in a fully fiber-integrated platform. Basis selection in our receiver for four-dimensional qudits is achieved using a single PFPM, substantially simplifying the receiver architecture. As a benchmark, we perform a four-dimensional QKD session and obtain a finite secret-key rate per pulse that, to the best of our knowledge, surpasses all previously reported QKD demonstrations. Our results establish poled-fiber electro-optic modulation as a broadly applicable platform for high-efficiency detection in fiber-integrated quantum information processing.

Comments: 9 pages, 5 figures. Please see the parallel paper "An optical-fibre-integrated buffer for packet-switched quantum networks'' that was submitted today to the arXiv

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Efficient detection of quantum states underpins advanced device-independent quantum-information protocols that provide the ultimate level of security for tasks including quantum random number generation and quantum key distribution (QKD). High-dimensional encoding is a natural route to boost the performance of such protocols, offering enhanced noise resilience and higher information capacity, yet their practical implementation remains challenging. A key experimental bottleneck in higher dimensions is the typical need of active modulators for basis selection, which incur substantial optical losses and polarization-sensitive operation. Poled optical fiber phase modulators (PFPMs) are a fiber-native electro-optic technology that naturally addresses these challenges, combining sub-dB insertion loss, intrinsic polarization independence, and direct compatibility with standard telecommunications fiber. Here we report the first use of a PFPM for active quantum-state measurements in a fully fiber-integrated platform. Basis selection in our receiver for four-dimensional qudits is achieved using a single PFPM, substantially simplifying the receiver architecture. As a benchmark, we perform a four-dimensional QKD session and obtain a finite secret-key rate per pulse that, to the best of our knowledge, surpasses all previously reported QKD demonstrations. Our results establish poled-fiber electro-optic modulation as a broadly applicable platform for high-efficiency detection in fiber-integrated quantum information processing.

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Quantum key distribution (QKD) provides information-theoretic security rooted in quantum physics, while high-dimensional (HD) encoding increases both noise tolerance and secret-key yield. Multicore fibers (MCFs), a leading platform for next-generation telecom networks, are a natural substrate for HD-QKD. Field demonstrations over deployed MCFs have so far relied on a hybrid qudit encoding strategy that combines two path (core modes) with the time-bin photonic degree of freedom, rather than exploiting the full set of available core modes. Although practical, this approach incurs intrinsic efficiency penalties that grow with dimension. Here we implement a four-dimensional (

d=4

) QKD protocol that directly exploits the full set of core modes of a four-core MCF, operating over an installed MCF network across the Universidad de Concepción campus under continuous environmental perturbations. We further benchmark the scheme using superconducting nanowire detectors at

10\,

dB channel loss, achieving a composable finite-key rate of

R = 6.19\times 10^{-3}\,

bits/pulse, the highest per-pulse rate reported to date for HD-QKD at comparable loss. This result establishes core-mode encoding as a viable architecture for realistic, high-rate quantum-secure communications.

We report a proof-of-principle quantum key distribution experiment using a one-way optical scheme with polarization encoding implementing the BB84 protocol. LiNbO 3 phase modulators are used for generating polarization states for Alice and active basis selection for Bob. This allows the former to use a single laser source, while the latter needs only two single-photon detectors. The presented optical scheme is simple and consists of standard fiber components. Calibration algorithm for three polarization controllers used in the scheme has been developed. The experiment was carried with 10 MHz repetition frequency laser pulses over a distance of 50 km of standard telecom optical fiber.